Quaternary Geology of Jefferson County, Wisconsin

Abstract

1 Abstract provides details about the surficial sediment and landscapes that were formed largely in the footprint of the Laurentide Ice Sheet’s southern Green Bay Lobe during the late Wisconsin Glaciation. These landscapes include lowlands underlain by glacial and postglacial lake sediment, uplands composed of streamlined till (drumlins), and meltwater-stream sediment. The southeastern corner of the county includes small sections of the Kettle Moraine, an interlobate zone that formed between the Green Bay and Lake Michigan Lobes of the Laurentide Ice Sheet. Map units are based on sedimentology, stratigraphic position, and landform association. All map units, except the Kettle Moraine and postglacial units, belong to the Horicon Member of the Holy Hill Formation. Older sediment is likely preserved in deep bedrock valleys that underlie the county. Glacial landforms associated with the Green Bay Lobe include active-ice features (drumlins and end moraines), meltwater features (eskers, ice-contact deltas, and outwash plains), and ice-disinte-gration or collapse features (kettles and hummocky topography). The southern Kettle Moraine in Jefferson County is composed of collapsed and uncollapsed meltwa-ter-steam sediment. Glacial and postglacial lake sediment is widespread in Jefferson County, including lake plains, sandy nearshore and shoreline landscapes. New radiocarbon ages from bulk plant materials in lake and wetland sediment indicate that the Green Bay Lobe had receded from Jefferson County by approximately 18,000 years before present and that lakes persisted into the Holocene before transitioning to wetlands by approximately 6,900 years before present. Holy Hill Horicon